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摘要
The Rapitan-type banded iron formation (BIF) in the Banda Alta Formation (Fm) of the Neoproterozoic Jacadigo Group in Brazil was deposited in a redox-stratified, marine sub-basin, which was strongly influenced by glacial advance/retraction cycles with temporary influx of continental freshwater and upwelling metal-enriched seawater from deeper anoxic parts. These new finding are based on new stratigraphic, whole-rock geochemical, and stable Fe and C isotope data from the "Santa Cruz" hematite deposit near Corumba, Mato Grosso do Sul, where a stratigraphy of lower and upper dolomite-rich and intermediate chert-rich BIF facies with up to three intercalated diamictites is revealed. The Ca-Mg-Fe-Mn-carbonate-chert-hematite and chert-hematite BIF (similar to 30-45 wt% Fe) of the lower dolomite-rich facies shows chemical signatures consistent with well-oxidized seawater, i.e. low (Pr/Yb)(PAAS), strong negative Ce-PAAS-, and positive Gd-PAAS- and Y-PAAS-anomalies, as well as negative-delta C-13 carbonate typical for Neoproterozoic glaciogenic carbonates. Sedimentation in a rather shallow water depth during relatively warm interglacial periods was likely influenced by abundant freshwater from fluviatile runoff and melting icebergs. In such conditions abundant microbial activity accommodated CO2 sequestration in carbonates as spheroids and mats and fractionated delta Fe-57 (-2.6 and 1.2%.) in primary Fe-hydroxides. In contrast, the intermediate chert-rich fades, characterized by chert-hematite BIF (similar to 35-55 wt% Fe) and isolated hematite chert and hematite mud, recorded trace element signatures of non-oxidized (absence of Ce anomalies and variable Y-PAAS-anomalies), more metal-enriched (Fe, Mn, Si, Ni, Zn, Pb, U) seawater, thus deposition was within and below the shallow-level redoxcline during ice cover. Colder water and isolation from sunlight reduced microbial activity, and thus an almost non-fractionated delta(57) Fe (-0.7 to 0.0%0) fluid signature reveals that low-temperature (based on almost absence of Eu anomaly in BIF) hydrothermal (MOR) vents or (sub-) seafloor alteration of mafic or felsic rocks, or shales fertilized seawater with metals. All results lead to the model that the Jacadigo Group formed during one major marine transgression-regression cycle, and BIF facies in the Banda Alta Fm were a response to first- and second-order periodic variations of the depth of the redoxcline, induced by the juxtaposition of glacial advance/retraction cycles, active graben tectonics, and glacial isostatic adjustment or eustatic water level changes. The chert-rich BIF facies marked the maximum of transgression. Age and tectonic setting of the Jacadigo basin remains contentious: it may represent a continental back arc basin of the Brasiliano collision zone (similar to 590 Ma), in which submarine alteration was possibly related to low-temperature hydrothermal fluids moving through the active graben system (as an alternative to basin-wide benthic pore water flux). This would place the Jacadigo group to the suite of Ediacarian "Gaskier" glaciations, although a relationship to the Marinoan glaciation (660-635 Ma), mainly based on the published sedimentation age-bracket of 706-587 Ma, is also possible.
- 出版日期2016-4
